204 East 15th Street
Eldon, Missouri 65026
Selection and synchonization of dual carbs on either a four or six cylinder inline engine
For best results, the very best thing is to do one’s homework BEFORE one purchases anything. There are several major criteria in the selection of one’s puchase, but the first criteria is how the unit will be used. Consider that 50~60 years ago, dual carb manifolds were sold for RACING! Today, while a very small percentage will submit their vehicles to the race track, most are interested in dual carbs for either LOOKS ONLY, or looks with a modest increase in performance. These two uses have vastly different requirements in carburetors.
Typically, the manifold sold in the period would be drilled for two carbs the same size as the original (a 216 Chevrolet would have 2 S. A. E. size 2 carbs, whereas a 235 Chevrolet would be drilled for 2 S.A.E. size 3 carbs). By using this arrangement, the user could order a second original carburetor from the dealer to go with original carb on the engine. At the same time, the user would install a full-race cam, a magneto ignition, headers, high-compression pistons; all to go with a numerical higher rear-end ratio BECAUSE THE ENGINE WOULD HAVE AT BEST A VERY HIGH IDLE, AND VERY LITTLE LOW END TORQUE. This is a good selection for a trailered race car, but a poor selection for the “cruiser” on the street.
Since the manifolds of today are basically copies of the manifolds of yesteryear, generally the street enthusiast will need to modify the manifold for smaller carburetors. Example: for a stock or modestly built Chevrolet 235, the very best carburetors would be those designed for a Chevrolet 216. The manifold would be drilled for size 3 carbs (2 15/16 inch mounting pattern), but the 216 carbs are size 2 (2 11/16 inch mounting pattern). There are three methods of doing the conversion: (A) use size 3 to size 2 adapters (extra cost, and looks “hokey”); (B) slot the holes in the carburetor (no additional cost, but looks even more hokey); or (C) have a good welder fill the original holes in the manifold, and then drill and tap for the correct size (extra cost, but looks much more like a factory installation, and weren’t looks the major criteria?).
If you plan to use on a trailered racecar, use the larger size.
The second criteria for selecting carburetors is that the EASIEST AND LEAST EXPENSIVE good results will be obtained by selecting either (A) carburetors which were originally designed for used in a multicarb environment (example: Carter WA-1’s used on the Twin-H Hudson) or (B) carburetors with a mechanical (as opposed to vacuum) power system. Carburetors with a vacuum power system may be used, but the tuning MAY be more difficult. If you have a spring winding kit and feel comfortable hand winding small precision springs, you may ignore this paragraph.
The next criteria is the one that gets the most arguments – the brand of carburetor to use. After almost 50 years of working on carburetors, we recommend Carters for most applications (readily available, reasonably priced, and parts readily available). For the high end user to whom cost is not an object, but perfection is the goal, we suggest either Strombergs are Zeniths. Other brands may be used, but there are no others that we recommend. In any event, one should choose a brand with which the user or the mechanic of the user is extremely familiar. In any event, ALWAYS use matched carburetors by tag numbers. Trying to use non-matched carburetors is simply asking for trouble.
If you start with used cores, acquire good rebuilding kits (we would suggest the ones we make); and rebuild the carburetors to absolutely stock calibration. By building to the stock calibration, you build a repeatable “baseline”. Calibration adjustments, if necessary, may be made later.
Once the carbs are rebuilt (or new carbs), one is ready for the installation and synchonization procedure, which is independant of brand, type, and size. Solid linkage will be required (unless you have some funky home-made manifold that has both carbs feeding into a centrally located plenum). Progressive linkage is not an option.
(A) Make certain that you have two IDENTICAL carburetors (check that the tag numbers are exactly the same, except for the production date), and that the carburetors being used are the approximate proper size for your application. If this condition is not true, stop reading and start looking!
(B) Screw the idle mixture control screws in on both carbs until they lightly "bottom". Check the carburetor manufacturer's spec for idle adjustment, and set both screws in the middle of the range. Example: if the spec is 1 to 2 turns, then use 1 1/2 turns.
(C) Set the throttle positioner screws higher than normal (so the engine will start and run at a high idle).
(D) Install both carburetors but NO linkage.
(E) Acquire a manometer. Uni-syn is a brand name that is readily available. Try the local motorcycle shop.
(F) Start the engine, and run at a high idle until the engine is at normal operating temperature, and that the chokes (if used) are completely off on both carburetors.
(G) Reduce the setting on the throttle positioner screws approximately an 1/8 of a turn at a time on each carb until the idle approximates desired RPM.
(D) Open the center adjustment on the Uni-Syn to the wide open position, and set the Uni-syn on the carburetor of your choice (it makes no difference). Adjust the control knob such that the plastic bobber is directly in the center of the column.
(E) Remove and replace the Uni-syn and verify that the engine does NOT change RPM with the addition/subtraction of the Uni-syn.
(F) Move the Uni-syn to the other carburetor. Adjust the throttle positioner screw of the second carburetor such that the plastic bobber is directly in the center of the column. When the carbs are synchronized, one should be able to move the Uni-syn from carb to carb with no change in RPM and no deflection in the position of the plastic bobber.
(G) If the idle is too high, again adjust the throttle positioner screws on both carbs to the desired RPM, and repeat the synchronization process.
(H) Install the linkage between the two carburetors. Make certain there is NO movement of the throttles.
(I) Install the linkage from the footfeed. Again make certain there is NO movement of the throttles.
(J) With the throttle linkage installed, verify the synchronization. If adjustment is needed, remove the linkage, and start over.
(K) Once synchronization is complete with throttle linkage in place, have an associate move the footfeed inside the passenger compartment to WOT, and visually ensure that the carburetor throttles also move to WOT.
(L) Its play time!